Ultra Low Frequency Low Power CMOS Oscillators for MPPT and Switch Mode Power Supplies

This paper presents the design of two low power consumption analog oscillators implemented in a 0.35 mu m CMOS technology. These oscillators were designed for a power conditioning circuit with an analogue Perturbation and Observation (P&O) Maximum Power Point Tracker (MPPT) to maximize the scavenged power generated by energy harvesting devices. The nominal frequency of the two oscillators is 15 Hz and 200 kHz, respectively. The 15 Hz oscillator is used to clock the MPPT, whereas the second oscillator generates a sawtooth wave required for the Pulse Width Modulation (PWM) of the switch mode converter. Both oscillators work with a supply voltage of 1 V and use a reference current generated by a self-biasing zero temperature coefficient circuit. All the circuitry was designed to operate in the sub-threshold region in order to keep its power consumption to a minimum. The frequency of the 15 Hz oscillator varies by 7.1% over a temperature variation from -40 degrees C to 125 degrees C. The total power consumption including the current reference circuit is 30 nW at 27 degrees C and reaches a maximum of 90 nW at 80 degrees C. The frequency of the 200 kHz oscillator varies by 33% over a temperature variation from -40 degrees C to 125 degrees C. The sawtooth generator, together with the current reference circuit, consume 63 nW across this temperature range.